We have purified and fully characterized two kinds of 5S ribosomal RNA genes from two species of Xenopus - X. laevis and X borealis. A nuclear extract support their faithful transcription in vitro. We will delete and mutate these genes by enzymatic and chemical methods, clone and characterize the deletions and test them for their ability to support initiation and termination of 5S RNA synthesis in the extract. The purpose of these experiments is to localize DNA regions that control the expression of the 5S RNA genes. The crude cell free extract will be fractionated to purify the factor(s) that influence accurate transcription. An assay system will be devised to reconstruct in vitro the developmental control that effects a shut off of oocyte 5S RNA genes in somatic cells. The DNA sequences responsible for this differential gene expression will be determined.